1. Field of the Invention
The present invention relates to the production of precipitated silicas and, more particularly, to a novel process for producing synthetic precipitated silicas having new and improved physical and chemical properties.
2. Description of the Prior Art
As known in the art, finely divided amorphous precipitated silicic acid pigments and certain zeolitic type alumino silicates may be prepared by the acidulation of an aqueous silicate solution with an acid or a salt of the acid, such as aluminum sulfate. Such products are commercially available being sold e.g., under the trademarks "Zeo"; "Zeolex" and "Arogen" by the J. M. Huber Corporation. Specific examples of these products as well as methods for their preparation are disclosed in U.S. Pat. Nos. 2,739,073; 2,848,346 and 3,582,379.
While the nature or characteristics of the above discussed precipitated silicic acid (sometimes herein referred to as silicas) and silicate pigments depend, in part, on the chemistry of the silicate solution (specifically the SiO.sub.2 /Na.sub.2 O ratio of the silicate) as well as the reaction conditions employed (precipitating pH, etc.), prior to the present invention such pigments were characterized by the following properties: high structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area and low pack density. In this regard, and due in part to the properties such as high oil absorption, high surface area, etc., the pigments have been widely and successfully used as reinforcing pigments in rubber, in paints, in the manufacture of paper, as moisture conditioners and the like.
However, and generally speaking for the moment, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are decreased thus increasing the overall cost in the production of the final product. For example, in the conventional production of silicic acid pigments as defined above the wet cake moisture content of the product (following filtration of the precipitated reaction mass) is approximately 82%. This means that there can be recovered only 18 parts of dry pigment from 100 parts of wet cake.